Abstract

Structural mapping has shown the North-South Axis at the western margin of the Pelagian Platform, to be dominated by NE/SW to N/S-trending anticlines and macro and mesoscale thrusts. The major anticlines have geometries consistent with fault-bend, tip-line and “hybrid” tip-line/fault-line folds and are interpreted to be underlain by “blind” thrusts with predominantly “ramp-flat” geometries and with decollement in Triassic evaporites. Restoration of NW-SE-orientated balanced cross-sections constructed across the North-South Axis indicates that up to 40 Km of shortening occurred in the Middle Miocence. Post-Miocene structures include sinistral and dextral strike-slip faults, normal faults, thrusts and folds. The North-South Axis is re-interpreted as a thin-skinned thrust belt, which forms the thrust front to the Atlas Mountains. As a consequence of this new interpretation the Alpine thrust belt in Tunisia is at least 100Km wider than proposed in previous models. The thrust front is oblique to the NE/SW-trending structures of the Atlas Mountains, which has generated a thrust belt with a northward taper. Spatial variations in the crustal rheology/thickness and/or thermal structure of the lithosphere, inherited from the Mesozoic Tethys passive margin in Tunisia, cannot explain this tapered geometry. Therefore, basement structures or related Mesozoic facies belts are believed to have controlled the orientation of the thrust front. Decompacted geohistory plots for wells in the Sahel E of the North-South Axis, show an increase in deposition rates synchronous with formation of the thrust front structures. This is interpreted as foreland basin subsidence. The Eocene to Lower Miocene sediments and Palaeofacies maps for central Tunisia are also consistent with a foreland basin/flexural bulge model suggesting that thrusting began as early as the Eocene in northern Tunisia.
The influence of Mesozoic Tethys passive margin structure on the geometry of the North-South Axis thrust belt is examined. Geohistory plots of wells from the Sahel and North-South Axis indicate variable Jurassic-Early Cretaceous sediment thickness. On interpreted seismic sections, these are shown to result from differential subsidence across NW/SE-trending oblique-slip or extensional faults and N/S-trending faults in the North-South Axis, situated above basement fractures. Late Cretaceous subsidence was generally slow and of probable thermal origin. However, Late Cretaceous strike-slip reactivation of the NW/SE-trending basement structures allowed the accumulation of thicker sediments in small strike-slip basins. The orientation of Mesozoic faults and Facies belts in central Tunisia indicate a NW/SE passive margin but with N/S-trending elements. In the North-South Axis, Middle Miocene lateral thrust structures are spatially related to Mesozoic sediment thickness variation, so that lateral thrust ramps are situates above NW/SE-trending Tethys basin margin faults. Similarly, the N/S trend of the thrust front might be controlled by N/S faults below the North-South Axis. Post-Miocene structures in the North-South Axis are consistent with sinistral wrenching on an underlying N/S basement fault. This suggest that basement fault reactivation controlled cover deformation after Middle Miocene thrusting. Sinistral wrenching in the postulated North-South Axis basement fault is consistent with a second order structure to the Medina wrench transform plate boundary to the NE.